Method for treating cover members for electrolytic cells



2,993,849 METHOD FOR TREATING COVER MEMBERS FOR ELECTROLYTIC CELLS Shannon W. Brown, Glendale, W. Va., assignor, by mesne assignments, to Pittsburgh Plate Glass Company No Drawing. Filed Apr. 17, 1958, Ser. No. 729,063 15 Claims. (Cl. 204-98) The present invention relates to the manufacture and operation of electrolytic cells. More particularly, the invention relates to a process for specially treating covers of electrolytic cells of the diaphragm type prior to use.

In the manufacture of chemicals by the electrolytic dissociation of various chemical compounds, many types of electrolytic cells such as the mercury cell, diaphragm cell, and others are employed. The diaphragm cell is one of the more common types of electrolytic cells and it is employed quite extensively in chemical industry, for example, in the manufacture of chlorine and caustic by the electrolytic dissociation of brine.

In operation of a process, such as the production of chlorine and caustic from brine, utilizing diaphragm type cells, it has been found that frequently, especially during the first several days of operation, cell flooding oc curs in many cell units. This flooding of cells undesirably causes shutdown of flooded cells with consequent losses in overall productivity.

The present invention, by suitably treating the cover members of diaphragm type electrolytic cells, inhibits or prevents this undesirable flooding and results in longer cell life and increased overall productivity.

Characteristic of the type of cell to which the invention pertains is the electrolytic cell described in U.S. Patent 1,866,065, granted July 5, 1932. Broadly, this cell has a bottom, usually constructed of concrete, within which there is positioned a lead slab. The anodes of the cell are embedded in the lead slab in spaced parallel relationship and rise vertically from it. Thecell cathode is constructed of a pliable material, such as Wire mesh, and is carried by an intermediate member, usually of steel construction. The cell cathode is formed into hollow projections or fingers so that When in place on the cell the cathode fingers rest in the spaces between the cell anodes. The hollow cathode fingers form the catholyte compartment of the cell and during cell operation the electrolyte, after contacting the cell anodes, flows through the cathode fingers to the catholyte compartment.

The cell diaphragm is constructed of a fibrous material such as asbestos and covers the wire mesh cathode fingers. As can be readily seen, when the cathode finers are in place on the cell there is formed two cell compartments, the anolyte compartment, which is the space formed between, the anodes, and the catholyte compartment, which is the spaced created in the hollow cathode fingers. The cover portion of the cell, containing a feed inlet and gas outlet, rests on the steel cathode frame and thus encloses the cell. The cover is generally of convex configuration on its outer surface and concave in configuration on its inner surface so that when in place it defines a gas space of considerable size between its inner surface and the brine level obtaining in the cell when brine is added to the cell.

Typically, cover members of diaphragm cells'of the type above referred to are constructed of material such as Portland cement, blast furnace slag cement, and the like, mixed with sand and stone aggregates to form a relatively strong concrete. The concrete may be suitably reinforced using steel rods, -wire mesh or other suitable reinforcing material.

Generally, in producing chlorine and caustic from "ice brine with a cell of this type, a plurality of such cells, arranged in rows or banks, are employed. The rows of cells are fed brine from a common feed line off of which individual feed conduits to individual cells are tapped. The brine as it enters the cell flows to the anolyte compartment first, and after partial dissociation, it flows through the cell diaphragm into the catholyte compartment where the electrolysis is completed.

During the operation of diaphragm cells of the above type, it has been observed that flooding often occurs. This is particularly so during the first several days operation when new cell covers are placed on the cells.

In operating cells of this type with new cell covers the incidence of flooding among a row of cells during the first week of operation has been as high as percent. It has also been observed that during operation the flow of electrolyte from the anolyte compartment through the diaphragm to the catholyte compartment within the cells becomes reduced on many of them in an operation of this type. Since the brine flowing into the cells is maintained at a more or less constant rate, the flow reduction within the individual cells causes a rise in the liquid level of the affected cells and eventual flooding of these cells occurs.

According to the instant invention it has been found that by exposing the aggregates on the inner surface of a cementitious cell cover prior to using such a cell cover on an electrolytic cell that flooding in diaphragm cells is retarded and in most instances completely eliminated. Treatments which expose the aggregates are conducted to provide aggregates on the inner surface of the cementitious cell covers in quantities such that from 30 to 9 percent of the inner surface is composed of aggregate.

Thus cementitious cell covers are formed by pouring concrete mix into a cover mold and permitting the concrete to harden. After hardening and before placing the molded article on a cell, the surface of the cover which is normally exposed to cell atmosphere during operation is treated to expose the aggregate at that surface. When the aggregate is exposed to an extent that it represents between 30 to 90 percent of the surface area, the treatment is stopped and the cover is then ready for use on an operating electrolytic cell.

Various techniques may be employed to accomplish the removal of cement from cover surfaces and consequent exposure of the aggregate. Thus in a preferred embodiment the inner cementitious surface of a new cell cover for a diaphragm type electrolytic cell is treated with an acid prior to using the cover on an operating electrolytic cell. While various acids may be employed, economics make it advisable to use aqueous solutions of strong mineral acids whose anions form Water soluble salts with the alkaline earth metal content of the cementitious surface. Acids suitable for such use are hydrochloric, nitric, hydrobromic, hydriodic, acetic, formic and the like.

The acid treatment of the inner cementitious surface of cell covers is such that a reaction with the surface is obtained. When the cover surface is contacted with the acid, a large portion of the cementitious material is chemically attacked, and upon Washing is removed. This removal of cementitious material exposes aggregates of the concrete on the treated surface of the cover. Thus, the inner surface of a cementitious cell cover when in operative position in an electrolytic cell presents to the atmosphere of the cell a surface comprised of exposed aggregates in high concentrations. When a cell cover, previously treated to expose the aggregates, is used on a diaphragm type cell, in place of a non-treated cover, it is found that cell flooding is entirely prevented or retarded to a substantial degree.

The concentration of acid suitable for providing exposure of aggregates is variable within the limits depending upon the length of time the cover is treated and the extent to which exposure of aggregate is desired. Similarly, the length of time that cell covers are exposed to the acid treatment will depend on the concentration of the acid and the amount of exposure desired.

Generally, aqueous acid solutions in concentrations of from about 1 percent to about 60 percent concentration by weight acid are acceptable for use in the process and the length of treatment may vary from as little as 15 minutes to as long as 48 hours or even longer. Preferably, this invention is practiced with aqueous mineral acid solutions having acid concentrations of between about 4 percent to about 20 percent concentration by weight and with treatments lasting from between 4 and 1 6 hours. These latter ranges give the desired degree of cover surface treatment and effect a removal of cementitious material to a depth of between A and A inch. The exact depth to which the cover is to be etched will depend in part upon the size and amount of aggregates below the inner cover surface.

After treating the cell cover with acid, the acid is removed and the cover surface is washed with water. Preferably, the treated surfaces are mechanically abraded or otherwise brushed to remove loose material, e.g., they are scrubbed with a stiff brush such as a wire bristle brush during the flushing or washing operation to facilitate removal of all loose cementitious material on the treated surface. After this operation, the surface water is removed and the cell cover is ready for use on an operating cell. In appearance the treated cover surface presents a larger amount of aggregate to cement than that which is obtained on the same surface prior to treatment. Generally, the inner cell cover surface presents no aggregates at all prior to treatment. This exposure is such that after treatment the amount of aggregates to total surface area is from about 30 percent to 90 percent while the same surface prior to treatment contained from about percent to 4 percent aggregates to total surface area.

While acid treatment forms a preferred method of treatment, other techniques may be utilized to accomplish the same result. Thus if desired, aggregate exposure may be accomplished by mechanical abrasion alone such as for example, by sand blasting, wire brushing and other similar techniques so long as the treatment results in a substantial exposure of aggregate on the inner cementitious cell cover.

The following example is given to illustrate one method which may be employed in carrying out the process of this invention.

Example A concrete mixture comprising the following was made:

1040 pounds of inch granite chips. 475 pounds of A1 to inch granite chips. 550 pounds of silica sand.

500 pounds of Portland cement.

The above ingredients were thoroughly mixed with water and poured into a mold of a diaphragm cell cover. Reinforcement was obtained using wire mesh and /2 inch steel reinforcing rods embedded in the concrete mix.

After the concrete had hardened, the cover was removed from the mold and placed onthe floor so that the cover surface which is exposed to the atmosphere of a cell during operation faced the ceiling. The gas outlet and brine inlet holes in the cover were tightly stoppered with rubber stoppers and 13.5 gallons of 33 percent by weight muriatic acid were introduced into the cover. Fifty-seven and one-half gallons of tap water were added to the acid in the cover, filling it completely.

The rmulting solution was held in contact with the cell covers surface in this manner at room temperature (25 C.) for 16 hours. At the end of 16 hours, the acid solution was removed from the cell top, the treated surface flushed with water and scrubbed with a steel brush. This treatment resulted in removing cementitious constituents from the contacted surface to a depth of 4 inch and provided a cover surface having aggregates representing about percent of the total surface area of the cover.

Six cell covers treated in this manner were placed in operative position on six diaphragm type electrolytic cells. These six cells were operated constantly under normal conditions at rated capacity. The operation was continued for a period of two weeks with no flooding occurring in the six cells during the entire period. Cells of this .type using new covers, which had not been treated, and

operated under the same conditions experienced flooding within one to three days.

While the present invention has been described with reference to the manufacture of chlorine and caustic from brine, it is not intended that the invention be restricted to this one process. It may be used, for example, in the production of chlorine and potassium hydroxide by the electrolytic dissociation of potassium chloride solutions and in other similar electrolytic processes without departing from the spirit and scope of the invention as set forth in the appended claims.

This application is a continuation-in-part of US. application Serial No. 683,719, filed September 13, 1957, now abandoned.

We claim:

1. A process for preventing cell flooding in an electrolytic cell of the diaphragm type for electrolyzing brine to produce chlorine and caustic and having a concrete cover member with a hardened inner concrete surface, said concrete surface being exposed in cell operation to the cell atmosphere, which process comprises prior to exposing the cover member to the cell atmosphere of an operating cell contacting the hardened inner concrete surface of said cover member with an aqueous acid solution to expose the aggregates at the hardened inner concrete surface, removing the acid, water washing and scrubbing the treated surface to remove material attacked by the acid, applying said cover to said cell and electrolyzing said brine.

2. The process of claim 1 wherein the aqueous acid solution employed is an aqueous solution of a mineral acid.

3. The process of claim 1 wherein the aqueous acid solution employed is an aqueous solution of an organic acid.

4. In the manufacture of chlorine and caustic by the electrolysis of brine in a diaphragm electrolytic cell, having a concrete cell cover the improvement comprising preparing said cover for use on said cell by contacting the hardened inner concrete surface of said cover with an acid to provide aggregates at said surface in amounts ranging between 30 and percent of the total area of said surface, said cover being removed from said cell during the acid contact, applying said cover to said cell and electrolyzing said brine.

5. A process for preventing flooding in an electrolytic cell of the diaphragm type having a concrete cover member with a hardened inner concrete surface said surface being exposed to the cell atmosphere during the operation of said cells to produce chlorine and caustic from brine which process comprises preparing said cover for use on said cell by contacting the surface of said cover with an aqueous acid solution to expose aggregates at said surface in amounts ranging between 30 and 90 percent of the total area of said surface, said cover being removed from said cell during contact with the acid solution, removing the acid solution and washing and scrubbing the treated surface to remove material attacked by the acid,

6. A process for preventing cell flooding in an electrolytic cell of the diaphragm type for electrolyzing brine to produce chlorine and caustic and having a concrete cover member with a hardened inner concrete surface, which is exposed during cell operation to cell atmosphere, which process comprises treating the hardened inner concrete surface of said cover to provide aggregate at said surface in amounts ranging between 30 and 90 percent of the total area of said surface, said cover being removed from said cell during the treatment, applying said cover to said cell and electrolyzing said brine.

7. In the manufacture of chlorine and caustic by the electrolysis of brine in a diaphragm type electrolytic cell having a concrete cover member with a hardened inner concrete surface said surface being exposed during cell operation to the cell atmosphere the improvement comprising treating the hardened inner concrete surface of said cover to expose aggregates at said surface in amounts ranging from between 30 and 90 percent of the total area of said surface, said cover being removed from said cell during the treatment, applying said cover to said cell and electrolyzing said brine.

8. In the operation of an electrolytic cell of the diaphragm type for electrolyzing brine to produce chlorine and caustic and having a concrete cover member with a hardened inner concrete surface which is exposed to cell atmosphere during cell operation the improvement comprising molding said cover member, permitting it to harden, after hardening treating the inner concrete surface of said cover to expose aggregates at said surface in amounts ranging from between 30 and 90 percent of 6 the total area of said surface, said cover being removed from said cell during said treatment, applying said cover to said cell and electrolyzing said brine.

9. The process of claim 4 wherein the acid employed in the process is a mineral acid.

10. The process of claim 4 wherein the acid employed in the process is an organic acid.

11. The process of claim 5 wherein the aqueous acid solution employed is an aqueous solution of a mineral acid.

12. The process of claim 5 wherein the aqueous acid solution employed is an aqueous solution of an organic acid.

13. The process according to claim 6 wherein the aggregates are exposed by abrasion.

14. The process of claim 7 wherein the treatment is accomplished by abrasion.

15. Process of claim 8 wherein treatment comprises mechanical abrasion.

References Cited in the file of this patent UNITED STATES PATENTS 1,361,763 Hamil Dec. 7, 1920 1,637,321 Johnson July 26, 1927 1,866,065 Stuart July 5', 1932 2,865,834 Ross Dec. 23, 1958 OTHER REFERENCES Construction Methods, August 1956, pp. 100-102. Industrial Chemist, April 1944, pp. 201-205. 

6. A PROCESS FOR PREVENTING CELL FLOODING IN AN ELECTROLYTIC CELL OF THE DIAPHRAGM TYPE FOR ELECTROLYZING BRINE TO PRODUCE CHLORINE AND CAUSTIC AND HAVING A CONCRETE COVER MEMBER WITH A HARDENED INNER CONCRETE SURFACE, WHICH IS EXPOSED DURING CELL OPERATION TO CELL ATMOSPHERE, WHICH PROCESS COMPRISES TREATING THE HARDENED INNER CONCRETE SURFACE OF SAID COVER TO PROVIDE AGGREGATE AT SAID SURFACE IN AMOUNTS RANGING BETWEEN 30 AND 90 PERCENT OF THE TOTAL AREA OF SAID SURFACE, SAID COVER BEING REMOVED FROM SAID CELL DURING THE TREATMENT, APPLYING SAID COVER TO SAID CELL AND ELECTROLYZING SAID BRINE. 